Opening on healers you can simply Garrote ->Kidney. With the exception that they are not playing with a warlock. (They can gate while silenced).

In that case you would want to do the more normal opener. Premed -> SnD then open with a cheap shot->Hemorrhage->ambush. with the 5 combo points you can either evis or kidney depending on the situation. then marked for death evis. This will do the most damage and give enough control to deal it.

The mage opener is Garrote->Cheap+Premed/Ambush -> Evis ->Marked for death Evis.

it is important to note that if the target is mounted cheap shot is always best, along with warlock gate.